The present invention relates to an automatic high insulation switch with a low consumption level and small overall dimensions. It relates to the field of the electrical measurement of currents having very low intensities, whose values are spread over several decades and, more specifically, relates to the case where said measurement is performed, in per se known manner, by means of an electrometric apparatus constituted by an integrated amplifier, preceded by at least one field effect transistor, having an insulated gate with a very low leakage current. The transistor-amplifier assembly is negatively fed back linearly by high value resistors, which are switched in accordance with the range of measurements to be performed.
For example, in the case of measuring very low currents with intensities between 10.sup.-4 and 10.sup.-7 amperes, from an ionization chamber, the values for the feedback resistors are then between 10.sup.6 and 10.sup.12 ohms, as a function of the range of measurement.
The invention more specifically relates to the switching device used for switching these high value resistors and which must have insulation characteristics compatible with these high values. The invention especially relates to known switching devices using magnetically controlled, flexible reed interrupters or switches enclosed in bulbs and known as reed bulbs or ILS interrupters or switches, connected in series with the different high value resistors used for the different ranges of measurement and which close when subject to the action of a magnetic field.
Hitherto, in the known switching devices of a first type, this magnetic field is produced by relays suffering from the disadvantage of having a relatively high consumption, due to their magnetic circuit, which must be imperfectly closed, in order to respect the leakage lines of the bulb switch or interrupter required for insulation purposes.
In the known switching devices of a second type, this magnetic field is produced by a permanent magnet which a manual control device positions in front of the bulb interrupter, whose contact is to be closed. This obviates the aforementioned disadvantage of a permanent high current consumption, but suffers from the disadvantage of using a manual control.